The present invention relates to an air-fuel ratio control system for an automotive engine, and more particularly to a system having an electronic fuel injection system controlled by a learning system.
In one type of electronic fuel-injection control, the quantity of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, intake-air pressure, engine load and engine speed. The quantity of fuel is determined by a fuel injector energization time (injection pulse width).
Generally, a desired injection amount is obtained by correcting a basic quantity of injection with various correction or compensation coefficients of engine operating variables. The basic injection pulse width T.sub.P is expressed, for example, as follows. EQU T.sub.P =K x Q/N
where Q is mass air flow, N is engine speed and K is a constant.
Desired injection pulse width T.sub.i is obtained by correcting the basic injection pulse T.sub.P with coefficients for engine operating variables. The following is an example of an equation for computing the actual injection pulse width. EQU T.sub.i =T.sub.p x.lambda.(Ka.times.COEF+K.sub.ACC -K.sub.DC)
where COEF is a miscellaneous coefficient comprising various correction or compensation coefficients obtained from memories dependent on coolant temperature and throttle position, .lambda. is a feedback correcting coefficient which is obtained from output signal of an O.sub.2 -sensor provided in an exhaust passage, and Ka is a correcting coefficient by learning (hereinafter called learning coefficient) for compensating the change of characteristics of devices with time in the fuel control system such as, injectors, and air flow meter employing hot wire, due to deterioration thereof, K.sub.ACC is an acceleration correction coefficient and K.sub.DC is a deceleration correction coefficient. The coefficients COEF, K, Ka, K.sub.ACC and K.sub.DC are stored in lookup tables and derived from the tables in accordance with sensed informations. The learning is executed in steady states of the engine operation. In order to detect the steady state, an operation matrix comprising a plurality of divisions is provided. The column and row of the matrix represent engine operating conditions such as engine speed N and basic injection pulse width T.sub.P. When the engine operating conditions, continue for a period of time within one of divisions, it is determined that the engine is in a steady state. In such a steady state, the learning operation is executed In the learning, the learning coefficient Ka corresponding to the engine operating conditions is rewritten with a new coefficient Ka*. The new coefficient Ka* is calculated by the following equation. EQU Ka*=Ka+M.times..DELTA.LMD
where .DELTA.LMD is a difference between an arithmetic average of maximum and minimum values in the output of O.sub.2 -sensor and a desired value in feedback control as a reference value, and M is a constant.
The learning is started when the output of the O.sub.2 -sensor changes cyclically, over a reference value for dividing a rich side and lean side, a predetermined number of times (three times) while the engine operating conditions stay in one of the divisions in the matrix.
During the idling of the engine, a short fuel injection pulse width is applied to the injectors so that a little change in the intake air flow causes a relatively large change in the pulse width. As a result, the air-fuel ratio changes largely. Accordingly, when the feedback control is carried out in the idling, the engine idling speed becomes irregular.
In addition, since the temperature of the engine decreases at idling, the output voltage of the O.sub.2 -sensor becomes low so that the amplitude thereof decreases. Therefore, a definite reference value can not be provided so that decision whether the air fuel is rich or lean becomes inaccurate. Thus, it is preferable to stop the feedback control during the idling.
On the other hand, during the steady state in idling, the learning operation is automatically executed. But the learning must be performed during the feedback control, because the feedback signal is used for the learning.
In order to meet such a requirement, Japanese Patent Laid Open 60-50246 discloses an air-fuel ratio control system where the feedback control is interrupted and the feedback correcting coefficient is held to a set value after the learning operation at the beginning of the idling state.
The interruption of the feedback control is performed only when the engine is in a steady state, and when the engine operating conditions stay in one of the divisions, for example the division of N.sub.2 -N.sub.3 /T.sub.P1 -T.sub.P2 of an operation matrix shown in FIG. 6a. However, when the altitude at which the vehicle is driven changes thereby changing the atmospheric density, or when an air-conditioner is used thereby increasing the engine load, the basic fuel injection pulse width T.sub.P varies. Therefore, the engine operating conditions may fluctuate in adjacent two division over the border line between the divisions as shown in FIG. 6b. Such a state cannot be detected as a steady state although it actually is. Accordingly, the feedback control is not stopped so that engine idle speed becomes irregular. In addition, the air-fuel ratio becomes overlean as a result of the drop of the output voltage of the O.sub.2 -sensor. Thus, engine idle speed is largely deviated from a desired engine speed. In order to detect that the engine is in a steady state under such conditions, the range of the divisions in the matrix must be enlarged. However, the learning dependent on such a large division causes aggravation of the air-fuel ratio control.